Pax9 is a member of the Pax family that consists of nine genes encoding nuclear transcription factors that play key roles in patterning and embryogenesis. In recent years, mouse and human genetic studies have provided definitive evidence of a key role for Pax9 in the patterning of dentition. Functional deletion of the Pax9 gene also results in an arrest in tooth development in mice. Recent studies on humans have associated mutations in PAX9, with autosomal dominantly inherited forms of tooth agenesis that primarily affects posterior teeth. Of the 11 PAX9 mutations reported thus far, 6 occur in the region of the paired domain that is encoded by exon 2 of PAX9. The latter is important in mediating the DNA-binding activities of PAX9. Apart from DNA-binding properties, however, relatively little is known about the role of Pax9 as a transcriptional regulator. Studies on other Pax family members point to the COOH terminal region that is characteristically rich in proline, serine and threonine as potentially mediating transactivation functions in vivo. We recently identified a novel frameshift mutation (793InsC) in the COOH-terminal domain (exon 4) of PAX9 in members of a large family with congenitally missing posterior teeth. Understanding the role of exon 4 will hence shed valuable insights into the molecular mechanisms underlying PAX9 functions during normal and abnormal tooth development. Two specific aims are proposed to test the fundamental hypothesis that the region of the COOH terminal that encompasses amino acid 264 (nt793) is critical for the transactivation function(s) of PAX9 and mutations in this region inactivate the transactivation domain and compromise PAX9-mediated transactivation of target genes during tooth morphogenesis. Aim 1 will delineate the transactivation function of Pax9 by identifying the minimal region of the COOH-terminal that yields maximal luciferase reporter-gene activity in cellular co-transfection assays. Aim 2 will evaluate the consequences of the 793InsC/amino acid 264 mutation on Pax9 transactivation and its protein structure. The role of the 793InsC/amino acid 264 Pax9 mutation will be investigated by determining alterations in transactivation of the 793InsC mutant protein relative to the wild-type. To complement the biochemical characterization of the differences in transactivation, circular dichroism will be utilized to investigate structural alteration in the COOH-terminal transactivation domain resulting from the 793InsC/amino acid 264 Pax9 mutation. Knowledge of the mechanism by which Pax9 transactivate target genes will facilitate a better understanding of the etiologic significance of mutations in PAX9 as well as provide insights into the regulation of transcription in eukaryotic cells. Understanding the biochemical function of Pax9 will ultimately provide a unique tool for the study of the genetic and molecular control of tooth morphogenesis.